Process and apparatus for separating nitroaromatics from wastewater

ABSTRACT

The invention relates to a process for the electrochemical treatment of aromatic nitro compounds, which comprises the steps: introducing an aqueous composition comprising at least one aromatic nitro compound into the anode space of an electrolysis cell and carrying out an electrolysis at an anodic current density in the range from 0.1 to 10 kA/m 2  and a cell potential in the range from 4 to 15 V.

The present invention relates to a process and an apparatus for the electrochemical treatment of aromatic nitro compounds, in particular an electrolytic process for treating alkaline process wastewater, e.g. from processes for the nitration of aromatic compounds to form mononitroaromatics, dinitroaromatics and trinitroaromatics. The aromatic nitro compounds comprised in the process wastewater and any nitrite comprised in the wastewater are reacted or destroyed by anodic oxidation or by means of anodically generated, oxidizing compounds. The process makes, in particular, complete oxidation of the aromatic nitro compounds to carbon dioxide and nitrate possible. The process can also be operated on a large scale and industrially.

Aromatic nitro compounds are usually prepared by nitration of the corresponding aromatic compounds (e.g. benzene, toluene, xylene, chlorobenzene) by means of a mixture of concentrated nitric acid and concentrated sulfuric acid, also referred to as nitrating acid. This forms an organic phase comprising the crude product of the nitration and an aqueous phase comprising essentially sulfuric acid, water of reaction and water introduced by the nitrating acid. The nitric acid used is generally largely consumed in the nitration.

After separation of the two phases, the aqueous, sulfuric acid-comprising phase is, depending on the technology of the nitration process, mixed again, either directly or after having been concentrated, with fresh nitric acid and used for the nitration. However, it is usually necessary to discharge at least part of the sulfuric acid, either continuously or discontinuously, from the overall process in order to avoid an increase in the concentration of impurities, in particular metallic salts (see, for example, DE 101 43 800).

The crude product from the nitration reaction of aromatic compounds (e.g. benzene, toluene, xylene, chlorobenzene) to form the corresponding nitroaromatics usually comprises the desired nitroaromatics (e.g. nitrobenzene (NB), dinitrobenzene (DNB), mononitrotoluene and dinitrotoluene (MNT and DNT), nitrochlorobenzene (NCB), nitroxylene) together with small amounts of aromatic nitro compounds which additionally have one or more hydroxyl and/or carboxyl groups.

These compounds are undesirable by-products. As undesirable by-products, it is possible for, for example, mononitrophenols, dinitrophenols and trinitrophenols (hereinafter also summarized as nitrophenols), mononitrocresols, dinitrocresols and trinitrocresols (hereinafter also summarized as nitrocresols), mononitroxylenols, dinitroxylenols and trinitroxylenols (hereinafter also summarized as nitroxylenols) and mononitrobenzoic and dinitrobenzoic acids (hereinafter summarized as nitrobenzoic acids) to be formed.

The crude product from the nitration has to be freed of the undesirable by-products before further use. The by-products are usually, after the aqueous phase (comprising sulfuric acid as nitrating acid) has been separated off, separated off by multistage washing of the organic phase with acidic, alkaline and neutral washing liquids, with washing generally being carried out in the order indicated. The alkaline washing is usually carried out using aqueous sodium hydroxide solution, aqueous sodium carbonate solution and/or aqueous ammonia solution.

The alkaline process wastewater formed comprises, inter alia, nitrophenols, nitrocresols, nitroxylenols and nitrobenzoic acids in the form of their water-soluble salts of the base used. They are usually present in a concentration of from 0.2 to 2.5% by weight, based on the alkaline process wastewater. The alkaline process wastewater also comprises neutral nitro molecules formed in the nitration, in particular reaction products. Neutral nitro molecules can usually be comprised in the alkaline process wastewater in an amount of greater than 1000 ppm. The alkaline process wastewater frequently also comprises from 500 to 5000 ppm of nitrates, from 500 to 5000 ppm of nitrite and more than 100 ppm of sulfate. These ions originate predominantly from the nitration. The constituents present give a typical chemical oxygen demand of from 1 to 20 g/l.

The nitrophenols, nitrocresols, nitroxylenols, nitrobenzoic acids and especially their salts have an intensive color and may be highly toxic to the environment. In addition, the nitrophenols, and especially their salts, are explosive in relatively high concentrations or in neat form and have to be removed from the wastewater before the latter is released. They are disposed of in such a way that they do not pose a risk to the environment. The alkaline process wastewater from the nitration of aromatics additionally comprises neutral nitro molecules formed in the nitration, in particular reaction produces. Since the aromatic nitro compounds can also have biocidal or bactericidal properties and thus make biological purification of the wastewater impossible, purification or work-up of the wastewater comprising aromatic nitro compounds is necessary.

This is also useful in order to be able to send the wastewater to a conventional wastewater treatment comprising a microbiological purification stage.

Numerous methods have been described in the literature for removing nitrophenols, nitrocresols, nitroxylenols, nitrobenzoic acids and the neutral nitroaromatics from process wastewater, for example from processes based on extraction, adsorption, oxidation or thermolysis.

The Encyclopedia of Chemical Technology, Kirk-Othmer, Fourth Edition 1996, Vol. 17, p. 138, describes an extraction process for separating off nitrobenzene, in which the nitrobenzene dissolved in the wastewater at the respective temperature is removed by extraction with benzene. Benzene which has dissolved in the water is removed by stripping before the final treatment of the wastewater.

EP-A 005 203 describes a thermal process for treating wastewater comprising hydroxy-nitroaromatics. Here, the wastewater comprising the hydroxynitroaromatics in the form of their water-soluble salts is heated to temperatures in the range from 150 to 500° C. under pressure with exclusion of air and oxygen.

The dissolved nitroaromatics and hydroxynitroaromatics can also be removed in an acid medium by extraction with an organic solvent (see Ullmanns Enzyklopädie der technischen Chemie, 4th edition, 1974, volume 17, page 386).

Furthermore, various methods for separating off organic nitro compounds, especially nitroaromatics from water and wastewater, which comprise a chemical or electrochemical reduction or oxidation step and optionally a further physical separation process (for example an extraction), are known.

DE-A 197 48 229 describes the electrochemical, cathodic reduction of nitrophenols and of nitroresorcinol to the corresponding amines at carbon cathodes. EP-A 808 920 describes the electrochemical reduction of nitrobenzene and nitrophenol at cathodes composed of various materials. In general, the known processes for the cathodic reduction of nitroaromatic compounds have the disadvantage that the nitroaromatics are not removed completely, or further process steps are required for separating off any environmentally damaging reaction products formed.

In addition, the above-described cathodic reduction processes often suffer from severe foaming in the cathode space, and the processes are therefore technically difficult to carry out, especially on a relatively large scale. A series of oxidative processes, e.g. nonelectrochemical, electrochemical or combined processes, have been described for the treatment of nitration wastewater or for the removal of aromatic nitro compounds.

U.S. Pat. No. 6,953,869 describes the removal of trinitrocresols and picric acid from nitrating acid by oxidation by means of concentrated nitric acid at temperatures of 70° C. U.S. Pat. No. 4,604,214 describes an oxidation process using Fenton's reagent for the oxidation of trinitrocresols and picric acid, in which an excess of oxidant is necessary and, in addition, complete removal of the aromatic nitro compounds is not possible. CN-A 1 600 697 describes the oxidation of p-nitrophenol with combined use of UV light, Fenton's reagent (H₂O₂/Fe(II)) and/or anodic oxidation at PbO₂ anodes.

The publication J. Hazardous Materials, Vol. 161, No. 2-3, 2009, pp. 1017-1023, describes a process for the electrochemical removal of dinitrotoluene and trinitrotoluene from the waste acid from the nitration of aromatics, in which the aromatic nitro compounds are oxidized by means of hydrogen peroxide which has previously been cathodically generated in situ from oxygen dispersed in the waste acid. The electrolytic process described here operated with introduction of oxygen and without separation of anode space and cathode space.

The publication “Proceedings of the 1992 Incineration Conference”, 1992, pp. 167-174, reports the indirect anodic oxidation of dinitrotoluene and trinitrotoluene in nitric acid solution by means of anodically generated Ag²⁺ ions. The process is carried out in a parallel plate reactor having platinum anodes and a Nafion cation-exchange membrane. The electrochemical process described is very sensitive to sulfate or halide ions and additionally involves the production of toxic carbon monoxide. In addition, complicated process steps are necessary for separating the silver ions from the liquid output.

CN-A 1 850 643 describes the removal of aniline and nitrobenzene from wastewater by means of an electrochemical process. Here, the aromatic nitro compounds are removed by oxidation in the anode space, with, in particular, an electrolysis apparatus comprising an anode comprising a titanium base material coated with ruthenium oxide, iridium oxide or lead oxide and also a cation-exchange membrane being used.

In general, the above-described, in part electrochemical oxidation processes have the disadvantage that additional complicated separation steps and an often complicated technology in terms of apparatus, e.g. introduction of gas, are necessary. In addition, a large excess of oxidant often has to be employed. It is usually not possible to achieve sufficient or complete removal of the environmentally damaging aromatic nitro compounds by the oxidation processes known from the prior art. In addition, the known processes and the apparatuses for carrying out the processes have an unsatisfactory period of uninterrupted operation.

The processes and apparatuses known from the prior art for removing aromatic nitro compounds from wastewater and process water also cannot ensure that simultaneous oxidation of a plurality of different aromatic nitro compounds is possible and that formation of possibly toxic degradation products does not occur.

It is therefore an object of the present invention to provide a process which can be carried out industrially using simple means and a suitable apparatus for carrying out an electrochemical oxidation of aromatic nitro compounds in aqueous compositions, and in particular a process for treating alkaline process wastewater from the nitration of aromatic compounds. In this process, complete or virtually complete oxidative degradation of the aromatic nitro compounds to form toxicologically acceptable compounds should be ensured.

The treated process water obtained should comprise no toxic, environmentally toxic and/or explosive substances and it should be able to be passed to a conventional wastewater treatment, including a microbiological purification stage. The wastewater should satisfy all present EU limits as a result of the process of the invention and/or as a result of a subsequent conventional wastewater treatment. Furthermore, the process should have stable operation and the apparatus should give a high process running time/operating time without interruption of the electrolysis cell. The process should be inexpensive and able to be realized in a simple fashion technically, also on a relatively large scale.

It has surprisingly been found that the process described below and the electrolysis apparatus described enable various nitroaromatic compounds which may be present in various forms (e.g. dissolved, emulsified and suspended) in an aqueous composition to be completely decomposed in an anodic oxidation using specific anodes at high anodic current density.

A further advantage of the electrochemical process of the invention is that the durability of the electrode materials, in particular the anode material, and of the separator material is maintained over a long period of time. Thus, long process running times of up to 2 years can be realized by means of the process described in the present invention and by means of the apparatus according to the invention. In particular, it has been able to be shown that stable operation of the process of the invention and of the apparatus of the invention for at least 750 hours is possible.

In addition, the foaming of the electrolyte composition and in particular of the anolyte caused by formation of gases, which usually represents a problem in the processes described in the prior art, is reduced or prevented completely in the process of the invention.

DE-A 10 2004 026 447 describes an electrolysis cell having a three-part structure for the removal of sulfate ions from water, with, inter alia, an anode coated with boron-doped diamond being used.

The present invention provides a process for the electrochemical treatment, in particular the electrochemical oxidation, of aromatic nitro compounds, which comprises the steps:

-   -   a) introducing an aqueous composition comprising at least one         aromatic nitro compound (hereinafter also referred to as         anolyte) into the anode space of an electrolysis cell,         -   where the electrolysis cell has at least one anode space and             at least one cathode space which are separated from one             another by a (at least one) separator,         -   and the electrolysis cell has at least one anode which             comprises at least one anode segment comprising (or             consisting of) platinum or an anode segment consisting of a             support material and a coating, where the support material             comprises at least one metal selected from the group             consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and             hafnium (Hf), but preferably comprises or consists of             niobium (Nb) and the coating consists of boron-doped             diamond;     -   b) carrying out an electrolysis at an anodic power density in         the range from 0.1 to 10 kA/m² and preferably a cell potential         in the range from 4 to 15 V, preferably in the range from 5 to 9         V.

In general, the inventive process described can be carried out in the entire pH range, i.e. both using acidic anolytes and using basic anolytes.

In a preferred embodiment, the invention relates to a process for the electrochemical treatment, in particular electrochemical oxidation, of aromatic nitro compounds as described above, with the aqueous composition (anolyte) being alkaline process wastewater from the nitration of aromatic compounds. Furthermore, the aqueous composition preferably has a pH in the range from 4 to 14, in particular in the range from 4 to 12, in particular from 4 to 10.

For the purposes of the present invention, aromatic nitro compounds are organic compounds which have a 6- to 14-membered aromatic ring, in particular a ring selected from among phenyl, naphthyl, anthracene and phenanthrene, to which at least one nitro group (—NO₂) is directly bound, with the aromatic ring also being able to have further substituents, in particular substituents selected from among C₁-C₆-alkyl, C₂-C₆-alkenyl, C₂-C₆-alkynyl, phenyl, benzyl, halo, —OH, —COOH and —COOR¹, where R¹═C₁-C₆-alkyl, C₂-C₆-alkenyl, C₂-C₆-alkynyl, phenyl or benzyl. The aromatic nitro compound preferably has a 6-membered aromatic ring.

In particular, the present invention provides a process for the electrochemical treatment, in particular the electrochemical oxidation, of aromatic nitro compounds of the general formula (I)

-   -   where:     -   n is an integer from 1 to 6;     -   m=0 or m is an integer from 1 to 5;     -   R is selected from among C₁-C₆-alkyl, C₂-C₆-alkenyl,         C₂-C₆-alkynyl, phenyl, benzyl, —F, —Cl, —Br, —OH, —COOH or         —COOR¹, where R¹═C₁-C₆-alkyl, C₂-C₆-alkenyl, C₂-C₆-alkynyl,         phenyl or benzyl.

In particular, the present invention provides a process for the electrochemical treatment of one or more of the above-described aromatic nitro compounds.

A preferred embodiment of the invention is a process for the electrochemical treatment of aromatic nitro compounds, wherein the aromatic nitro compounds are at least one compound selected from the group consisting of nitrobenzene (NB), dinitrobenzene (DNB), trinitrobenzene (TNB), mononitrotoluene (MNT), dinitrotoluene (DNT), trinitrotoluene (TNT), nitrochlorobenzene (NCB), mononitroxylenes, dinitroxylenes, trinitroxylenes, mononitrocresol, dinitrocresol, trinitrocresol, mononitrophenol, dinitrophenol, trinitrophenol, mononitrobenzoic acid, dinitrobenzoic acid, trinitrobenzoic acid, mononitroxylenols, dinitroxylenols and trinitroxylenols, with all isomeric forms of the compounds mentioned being encompassed.

Aromatic nitro compounds which do not comprise a hydroxyl group or carboxyl group in the molecule will also, for the purposes of the invention, be referred to as “neutral nitro molecules” or “neutral nitro aromatics. Nitrophenols, nitrocresols, nitroxylenols and nitrobenzoic acids will hereinafter also be summarized as hydroxynitroaromatics. In a preferred embodiment, the aromatic nitro component is a mixture of at least two of the abovementioned compounds.

The aromatic nitro compound(s) can be present in dissolved, emulsified or suspended form in the aqueous composition. In particular, the invention provides a process as described above for the electrochemical treatment, in particular the electrochemical oxidation, of aromatic nitro compounds, where the aromatic nitro compound(s) is/are present in at least two of the abovementioned forms.

Preference is also given to a process as described above, wherein the aqueous composition comprises (optionally in addition to at least one dissolved nitro compound) at least one aromatic nitro compound, preferably in suspended form.

In a preferred embodiment of the invention, the aqueous composition (anolyte) comprises at least one aromatic nitro compound in an amount in the range from 0.1 to 5% by weight, preferably in the range from 0.5 to 2.5% by weight (based on the total aqueous composition).

In a preferred embodiment of the invention, the aqueous composition comprises not only the aromatic nitro compound but also further additives, in particular in a concentration of from 0.001 to 30 g/l, preferably in a concentration of from 0.01 to 10 g/l. The aqueous composition can also comprise inorganic nitrites in addition to the aromatic nitro compound.

As further additives, water-soluble salts can be added to the aqueous composition comprising at least one aromatic nitro compound in order to increase the conductivity. These salts are selected, in particular, from among water-soluble inorganic salts, in particular salts comprising nitrate, sulfate and/or carbonate, in particular alkali metal salts comprising nitrate, sulfate and/or carbonate. The abovementioned salts for increasing the conductivity can be comprised, in particular, in a concentration in the range from 0.1 to 30 g/l, preferably in the range from 0.1 to 10 g/l, particularly preferably in the range from 1 to 10 g/l (based on the aqueous composition) in the aqueous composition.

For the purposes of the present invention, a water-soluble salt is generally a salt having a solubility in water of greater than or equal to 1 mol/l.

The present invention further provides a process as described above, wherein the aqueous composition (anolyte) comprising at least one aromatic nitro compound additionally comprises a (at least one) redox mediator (e.g. an alkali metal sulfate).

The redox mediator can be comprised, in particular, in a concentration in the range from 0.001 to 0.2 mol/l, in particular in the range from 0.01 to 0.05 mol/l (based on the aqueous composition), in the aqueous composition.

The redox mediator can be, in particular, at least one compound selected from among inorganic salts comprising sulfate ions, in particular an alkali metal or alkaline earth metal sulfate; and inorganic salts of cerium (Ce) or praseodymium (Pr), in particular nitrates, sulfates and/or hydrogenphosphate salts of cerium or praseodymium.

In one embodiment of the invention, the aqueous composition (anolyte) comprises from 0.1 to 10 g/l, preferably from 1 to 5 g/l, of cerium and/or praseodymium ions.

As catholyte, particular preference is given to using a solution selected from the group consisting of lithium hydroxide solution, sodium hydroxide solution, potassium hydroxide solution and ammonium sulfate solution.

The concentration of the catholyte solution is preferably from 0.1 to 5 mol/l, particularly preferably from 0.5 to 2 mol/l. In particular, it is possible to use an ammonium sulfate solution having a concentration in the range from 0.5 to 2.5 mol/l as catholyte.

The process of the invention can, in particular, be carried out so that the temperature of the aqueous composition in the anode space during the electrolysis is in the range from 30 to 90° C., particularly preferably in the range from 40 to 70° C.

The duration of the electrolysis can preferably be in the range from 0.3 to 10 hours, in particular from 1 to 10 hours, preferably in the range from 1 to 5 hours. The process described can be operated in the continuous mode, in a mode with partial recycling of the process stream, e.g. with a recycling ratio in the range from 80 to 98%, preferably in the range from 90 to 95%, or batchwise.

The present invention provides a process as described above, in which the electrolysis cell has at least one anode space and at least one cathode space which are separated from one another by a separator and the separator is selected from among

-   -   unspecific separators based on inorganic or organic porous         materials (e.g. separators made of sintered silica);     -   cation-exchange membranes based on polyethylene composite         polymers and/or polyvinyl chloride composite polymers and/or         polyvinylidene fluoride (PVDF) and/or polytetrafluoroethene         (PTFE), in particular comprising sulfonate groups (e.g. MC-3470,         manufacturer Ionac)     -   strongly basic anion-exchange membranes, in particular on the         basis of composite polymers, e.g. the abovementioned polymers,         comprising tertiary amino groups (e.g. MC-3450 from the         manufacturer Ionac).

The anode of the electrolysis cell used in the process of the invention preferably comprises a base frame to which at least one anode segment is fastened (e.g. screwed), with the base frame comprising a metal which has a high overvoltage for the formation of oxygen and is electrochemically passive under the given conditions.

In particular, the base frame can comprise one or more “valve metals”. For the present purposes, a valve metal is a metal which when connected as anode becomes coated with a semiconducting (insulating) oxide layer which does not become conductive even at a high overvoltage and thus blocks the electrolysis. As valve metals, mention may be made of, inter alia, niobium, tantalum, titanium, hafnium, zirconium and tungsten.

The base frame preferably comprises at least one metal selected from among titanium (Ti), niobium (Nb), tantalum (Ta) and hafnium (Hf). It is also possible to use alloys of the metals mentioned.

A preferred embodiment of the invention provides a process as described above, wherein the electrolysis cell has an anode which comprises at least one anode segment comprising a support material and a coating,

-   -   where the support material comprises at least one valve metal,         preferably at least one metal selected from the group consisting         of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf),         particularly preferably niobium (Nb), and the coating comprises         boron-doped diamond,     -   and the electrolysis is carried out at an anodic current density         in the range from 0.1 kA/m² to 2 kA/m², preferably in the range         from 0.1 to 1.25 kA/m², preferably in the range from 0.75 to         1.25 kA/m², and a cell potential in the range from 4 to 15 V,         preferably in the range from 5 to 9 V.

In a preferred embodiment, the above-described anode segment comprising a support material and a coating has a coating comprising boron-doped diamond in an amount of from 90 to 100% (based on the electrochemically active anode area or based on the electrochemically active area of the anode segment/segments). Preference is given to using anode segments which have a corresponding coating on two sides, preferably on all sides. In particular, the boron-doped diamond has a dopant content of from 0.01 to 3%, in particular from 0.1 to 0.5%.

In a further embodiment, the above-described anode segment (comprising a support material and a coating) has a coating of boron-doped diamond in a layer thickness in the range from 5 to 50 μm, in particular from 10 to 40 μm. The support material comprising a metal selected from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf), in particular comprising niobium (Nb), preferably has a thickness of from 1 to 4 mm, in particular from 2 to 3 mm.

An embodiment of the invention provides a process for the electrochemical treatment of aromatic nitro compounds as described above, wherein the electrolysis cell has an anode comprising at least one anode segment comprising a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf); where the coating comprises boron-doped diamond, the anode segment is fastened to a base frame comprising at least one metal selected from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) and the electrolysis is carried out at an anodic current density in the range from 0.1 kA/m² to 2 kA/m² and a cell potential in the range from 4 to 15 V and the coating comprising boron-doped diamond on the support material has a layer thickness in the range from 5 to 50 μm.

In the above-described embodiment of an anode comprising at least one anode segment comprising a support material and a coating, the base frame is preferably almost completely or completely covered by the anode segment or the anode segments. In particular, a coverage of the base frame by the anode segment or the anode segments can, in this embodiment, also be greater than 100%. In this embodiment, the base frame can, in particular, not be manufactured with a solid area and can, for instance, comprise a grid of metal strips which are electrically conductive and mechanically stably joined to one another (e.g. screwed, welded). In particular, the actual anode segments can then be fastened to these metal strips. The base frame grid structure preferably has on its rear side contact pins or contact tabs for the supply of electric power and can, in particular, be fixed in the anode shell by means of the power leads.

A further preferred embodiment of the invention provides a process as described above, wherein the electrolysis cell has an anode which comprises at least one anode segment comprising or consisting of platinum.

In particular, the invention provides a process as described above, wherein the electrolysis cell has an anode which comprises at least one anode segment consisting of platinum,

-   -   and the electrolysis is carried out at an anodic current density         in the range from 0.1 to 10 kA/m², preferably in the range from         2.5 to 10 kA/m², preferably in the range from 4 to 10 kA/m²,         particularly preferably in the range from 2 to 6 kA/m², and a         cell potential in the range from 4 to 15 V, preferably in the         range from 5 to 9V.

The electrolysis is preferably carried out at a platinum segment electrode as described above at an anodic current density above 2.5 kA/m², in particular above 4 kA/m².

The electrolysis cell can preferably have an anode which comprises at least one anode segment comprising or consisting of platinum,

-   -   wherein the anode segment or the anode segments is/are in the         form of foils, sheet or woven wire mesh and is/are fastened to a         base frame comprising at least one metal selected from among         niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf),     -   and the anode segment or anode segments cover(s) the surface of         the base frame to an extent of not more than 30%, preferably not         more than 10%.

In particular, coverage of the base frame by the above-described electrochemical active anode segments in the range from 1 to 30%, preferably in the range from 1 to 10%, is advantageous.

In particular, at least one anode segment comprising finely polished pure platinum is used in the preferred embodiment. Such platinum electrodes are known to those skilled in the art and are commercially available. A smooth, finely polished platinum electrode can typically be obtained by polishing using a silica gel polishing composition having a particle size of about 0.05 μm.

As coproduct of the electrolysis, hydrogen, in particular, is obtained in a high purity. This hydrogen can be used in other processes, e.g. in the further processing of the aromatic nitro compounds by catalytic reaction (reduction).

Furthermore, the present invention provides an apparatus for the electrochemical treatment, in particular the electrochemical oxidation, of aromatic nitro compounds, which comprises at least one electrolysis cell

-   -   wherein the electrolysis cell has at least one anode space and         at least one cathode space which are separated from one another         by a (at least one) separator     -   and the electrolysis cell has at least one anode which comprises         at least one anode segment comprising (or consisting of)         platinum or an anode segment comprising a support material and a         coating, where the support material comprises (or consists of)         at least one metal selected from the group consisting of niobium         (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) and the         coating comprises boron-doped diamond.

As separator, it is possible to use both specific and unspecific separators. For this purpose, it is possible to use, for example, cation-exchange membranes, organic porous materials, inorganic porous materials and/or strongly basic anion-exchange membranes. In a preferred embodiment, the separator is selected from among:

-   -   unspecific separators based on inorganic or organic porous         materials (e.g. separators made of sintered silica);     -   cation-exchange membranes based on polyethylene composite         polymers and/or polyvinyl chloride composite polymers and/or         polyvinylidene fluoride (PVDF) and/or polytetrafluoroethene         (PTFE), in particular comprising sulfonate groups (e.g. MC-3470,         manufacturer Ionac)     -   strongly basic anion-exchange membranes, in particular on the         basis of composite polymers, e.g. the abovementioned polymers,         comprising tertiary amino groups (e.g. MC-3450 from the         manufacturer Ionac).

The invention also provides a process in which the electrolysis cell has an anode comprising at least one anode segment comprising a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium, tantalum, titanium and hafnium; the coating comprises boron-doped diamond; the anode segment is fastened to a base frame comprising at least one metal selected from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) and the electrolysis is carried out an anodic current density in the range from 0.1 kA/m² to 2 kA/m² and a cell potential in the range 4-15 V and the coating of boron-doped diamond on the support material has a layer thickness in the range from 5 to 50 μm.

The invention also provides an apparatus for the electrochemical treatment of aromatic nitro compounds, which comprises at least one electrolysis cell, wherein the electrolysis cell has at least one anode space and at least one cathode space which are separated from one another by a separator.

Here, the electrolysis cell has at least one anode which comprises at least one anode segment comprising platinum or an anode segment comprising a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) and the coating comprises boron-doped diamond.

The invention also provides an apparatus in which the separator is selected from among unspecific separators based on inorganic or organic porous materials, cation-exchange membranes based on polyethylene composite polymers and/or polyvinyl chloride composite polymers and/or polyvinylidene fluoride (PVDF) and/or polytetrafluoroethene (PTFE) and anion-exchange membranes.

In particular, it has been found to be advantageous to provide a spacer between the anode and the separator, which separator ensures maintenance of a sufficient spacing between anode and separator in order to ensure protection of the separator from the strong oxidants formed in the anode reaction (e.g. hydroxyl radicals, perhydroxyl radicals, peroxodisulfate anions, Caro's acid and ozone). In particular, the spacing between anode and separator can be in the range from 2 to 12 mm, preferably from 4 to 8 mm. This enables damage to the separator (the membrane) by the strong oxidants to be avoided.

One embodiment of the invention is an apparatus as described above, wherein the electrolysis cell has a spacer between the separator and the anode, where the spacing between anode and cathode is in the range from 2 to 12 mm, preferably in the range from 4 to 8 mm.

In particular, the spacer between anode and cathode can be a multilayer woven mesh, preferably a three-layer, three-dimensional woven mesh (gauze), composed of nonconductive polymer, with the various layers having a different, in particular gradated, aperture (proportion of open area). The spacer used is preferably composed of an oxidation-resistant, electrically nonconductive polymer, where the oxidation-resistant, electrically nonconductive polymer is selected from among polyesters and polyalkylene (e.g. polyethylene, polypropylene).

In a particular embodiment of the invention, the spacer has three different layers having a different aperture (proportion of open area), with the layers of the spacer adjacent to the anode or to the separator having an aperture in the range from 50 to 90%, preferably from 60 to 75%, and the middle layer of the spacer having an aperture of from 30 to 60%, preferably an aperture of from 40 to 60%. This gradation of the aperture of the spacer can, in particular, aid establishment of turbulent flow close to the anode.

In particular, the apparatus of the invention comprises a cell body formed by at least one cathode space and at least one anode space in the numbers required in each case, provided with connections for introduction and discharge of the anolyte and catholyte and also sealing systems, pressure frames and power supply leads for the electrodes, with selection and arrangement being known to those skilled in the art. Preference is given to a two-chamber electrolysis cell or a three-chamber electrolysis cell.

The electrolyte preferably flows upward, i.e. from the bottom toward the top, through the electrolysis cell.

The cathode preferably comprises a sufficiently corrosion-resistant metal or carbon (in the form of graphite or glassy carbon). The cathode is preferably made of stainless steel sheet. In particular, the cathode material is resistant to embrittlement by hydrogen.

Furthermore, the present invention provides for the use of an apparatus as described above for treating wastewater or process water, in particular from the nitration of aromatic compounds, in particular alkaline process water.

The invention is illustrated by the following examples.

EXAMPLE 1

An aqueous process solution from a chemical production process, which was characterized as follows:

-   -   1.5 g/l of NaNO₂     -   20 g/l of Na₂CO₃     -   10 mg/l of mononitrotoluene isomer mixture with a proportion         emulsified     -   1 g/l of dinitrotoluene isomer mixture with a proportion         suspended     -   0.5 g/l of trinitrocresol isomer mixture with a proportion         suspended         was used.

100 ml of the aqueous process solution described were introduced into the anode space of an electrolysis cell, while a sodium hydroxide solution having a concentration of 1 mol/l was present in the cathode space. The electrode spaces were separated by means of a diaphragm composed of sintered silica having a pore opening of from 30 to 50 μm. A smooth platinum sheet having a size of 1.5 cm² and a thickness of 0.25 mm was used as anode. After application of a DC voltage, the solution was electrolyzed at an anodic current density of 0.6 A/cm² and a temperature of 70° C.

The electrolysis process was carried out batchwise. After an electrolysis time of four hours, the previously dark brown liquid was clear and decolorized.

The chemical oxygen demand of the solution was reduced from 6500 mg/l (original process solution) to 150 mg/l, corresponding to a conversion of 98%. Nitrite was no longer detectable in the solution after the end of the electrolysis. The chemical oxygen demand was determined in accordance with DIN 38409/H41.

EXAMPLE 2

A process solution as described in example 1 was introduced into an electrolysis cell in which a 4 cm² anode composed of boron-doped diamond on niobium sheet having a thickness of 1.5 mm was used as anode. The electrolysis was carried out at a temperature of 60° C. and an anodic current density of 0.125 A/cm². The experimental setup otherwise corresponded to the setup as described in example 1. The electrolysis process was carried out batchwise. After an electrolysis time of two hours, the solution in the anode space was completely clear and colorless. The chemical oxygen demand had been reduced from an initial 6500 mg/l to below 50 mg/l, corresponding to a conversion of more than 99%. Nitrite was no longer detectable in the solution from the anode space.

EXAMPLE 3

An aqueous process solution comprising:

-   -   1.0 g/l of sodium nitrite     -   and also mononitrotoluene, dinitrotoluene, nitrocresols,         and having a chemical oxygen demand (COD) of 5100 mg/l was used.

An electrolysis cell having an anode comprising a titanium/auxiliary frame with a boron-doped diamond electrode on a niobium support fastened thereto was used. The anode area was 75 cm².

Furthermore, the electrolysis cell comprised a spacer in the form of a woven polymer mesh having 3 layers of different aperture in the anode space. The layers of the spacer adjacent to the anode and to the separator had an aperture in the range from 60 to 75% and the middle layer of the spacer had an aperture of from 40 to 60%. The spacing between anode and separator was 6.5 mm.

An MC/3470 cation-exchange membrane (manufacturer Ionac, USA) and a cathode made of CrNiTi stainless steel were used. The cathode area was 100 cm². A 1M sodium hydroxide solution was used as catholyte. 2.5 l of the abovementioned process solution were electrolyzed batchwise at a current of 10 A and a temperature of 70° C.

The electrolysis process was carried out batchwise and with partial recycling in the range from 80 to 95%, with the results described below being similar for the modes of operation. After an electrolysis time of four hours, a chemical oxygen demand of 600 mg/l was measured on a sample from the anode space, corresponding to a conversion of 88%.

During the electrolysis, 150 ml of the liquid went over from the anode space into the cathode space. The chemical oxygen demand (COD) of the catholyte was 40 mg/l after the end of the electrolysis (0.0 mg/l before the beginning of the experiment).

EXAMPLE 4

An aqueous process solution as described in example 1 was used; the electrolysis cell as described in example 3 was employed.

Sodium sulfate was additionally added to the liquid in the anode space in such an amount that a concentration of 5 g/l based on the total amount of liquid was obtained. The electrolysis process was carried out batchwise with partial recycling in the range from 80 to 95%; the results described below for the mode of operation were similar. The electrolysis was carried out at a current of 10 A and a temperature of 60° C. After an electrolysis time of four hours, the chemical oxidation demand of the liquid in the anode space was found to be 360 mg/l, corresponding to a conversion of 94%.

EXAMPLE 5

The electrolysis was carried out as described in example 4, with the difference that praseodymium nitrate instead of sodium sulfate was added to the liquid in the anode space such that a concentration of praseodymium ions in the liquid in the anode space of 0.005 mol/l was obtained. The electrolysis process was carried out batchwise with partial recycling in the range from 80 to 95%; the results described below for the mode of operation were similar. After an electrolysis time of 3.5 hours, the chemical oxygen demand had been reduced to 270 mg/l, corresponding to a conversion of 96%.

EXAMPLE 6

An aqueous process solution comprising nitrotoluene and nitrocresols and having a chemical oxygen demand of 7200 mg/l was used. The electrolysis cell comprised anode, cathode and spacer as described in examples 4 and 5. An MA/3450 anion-exchange membrane (manufacturer Ionac, USA) served to separate the electrode spaces. A 2 M ammonium sulfate solution was used as catholyte. The electrolysis process was carried out batchwise with partial recycling in the range from 80 to 95%; the results described below for the mode of operation were similar. After an electrolysis time of 1 hour at a current of 10 A, the value for the chemical oxygen demand (COD) of the liquid flowing out of the anode space was 150 mg/l. An increase in volume of the liquid in the cathode circuit was not observed. The value for the chemical oxygen demand of the catholyte over the total time of the electrolysis experiment was 1 mg/l.

EXAMPLE 7

An aqueous process solution comprising aromatic nitro compounds and having a chemical oxygen demand of 6000 mg/l was used. A 2 M ammonium sulfate solution was used as catholyte. An electrolysis cell having an anode area of 600 cm² was used. A boron-doped diamond anode was used as anode. The electrolysis process was carried out continuously at a current density of 1 kA/m², with an anolyte volume flow of 1.5 l/h and at 70° C. A conversion based on the COD of the anolyte of 89% was able to be achieved. 

1. A process for the electrochemical treatment of aromatic nitro compounds, which comprises the steps: a) introducing an aqueous composition comprising at least one aromatic nitro compound into the anode space of an electrolysis cell, where the electrolysis cell has at least one anode space and at least one cathode space which are separated from one another by a separator, and the electrolysis cell has at least one anode which comprises at least one anode segment comprising platinum or an anode segment consisting of a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf), and the coating consists of boron-doped diamond; b) carrying out an electrolysis at an anodic power density in the range from 0.1 to 10 kA/m² and a cell potential in the range from 4 to 15 V.
 2. The process according to claim 1, wherein the aqueous composition is alkaline process wastewater from the nitration of aromatic compounds.
 3. The process according to claim 1, wherein the aromatic nitro compounds are at least one compound selected from the group consisting of nitrobenzene (NB), dinitrobenzene (DNB), trinitrobenzene (TNB), mononitrotoluene (MNT), dinitrotoluene (DNT), trinitrotoluene (TNT), nitrochlorobenzene (NCB), mononitroxylenes, dinitroxylenes, trinitroxylenes, mononitrocresol, dinitrocresol, trinitrocresol, mononitrophenol, dinitrophenol, trinitrophenol, mononitrobenzoic acid, dinitrobenzoic acid, trinitrobenzoic acid, mononitroxylenols, dinitroxylenols and trinitroxylenols, with all isomeric forms of the compounds mentioned being encompassed.
 4. The process according to claim 1, wherein the aqueous composition comprises at least one aromatic nitro compound in suspended form.
 5. The process according to claim 1, wherein the aqueous composition comprising at least one aromatic nitro compound additionally comprises a redox mediator.
 6. The process according to claim 1, wherein the temperature of the aqueous composition in the anode space in the electrolysis is in the range from 30 to 90° C.
 7. The process according to claim 1, wherein the electrolysis cell has an anode which comprises at least one anode segment comprising platinum and the electrolysis is carried out at an anodic current density in the range from 2.5 kA/m² to 10 kA/m² and a cell potential in the range from 4 to 15 V.
 8. The process according to claim 1, wherein the electrolysis cell has an anode which comprises at least one anode segment consisting of platinum, wherein the anode segment is in the form of foils, sheet or woven wire mesh and is fastened to a base frame comprising at least one metal selected from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf), and the anode segment covers the surface of the base frame to an extent of not more than 30%.
 9. The process according to claim 1, wherein the electrolysis cell has an anode comprising at least one anode segment comprising a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf); the coating comprises boron-doped diamond; the anode segment is fastened to a base frame comprising at least one metal selected from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) and the electrolysis is carried out an anodic current density in the range from 0.1 kA/m² to 2 kA/m² and a cell potential in the range from 4 to 15 V and the coating of boron-doped diamond on the support material has a layer thickness in the range from 5 to 50 μm.
 10. An apparatus for the electrochemical treatment of aromatic nitro compounds, which comprises at least one electrolysis cell, wherein the electrolysis cell has at least one anode space and at least one cathode space which are separated from one another by a separator, and the electrolysis cell has at least one anode which comprises at least one anode segment comprising platinum or an anode segment comprising a support material and a coating, where the support material comprises at least one metal selected from the group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf) and the coating comprises boron-doped diamond.
 11. The apparatus according to claim 10, wherein the separator is selected from among unspecific separators based on inorganic or organic porous materials, cation-exchange membranes based on polyethylene composite polymers and/or polyvinyl chloride composite polymers and/or polyvinylidene fluoride (PVDF) and/or polytetrafluoroethene (PTFE) and anion-exchange membranes.
 12. The apparatus according to claim 10, wherein the electrolysis cell has a spacer between the separator and the anode, where the spacing between anode and cathode is in the range from 2 to 12 mm.
 13. The apparatus according to claim 10, wherein the spacer between anode and cathode is a multilayer woven mesh composed of nonconductive polymer, with the various layers having a gradated aperture. 